A spinnaker sail has evolved through a number of developments. These developments have in part been associated with the developments in spinnaker sail material and in part from a better understanding how this sail has to function under optimum performance conditions. Although some spinnaker sails have been designed specifically for running conditions, i.e. for apparent wind angles of about 110.degree. to 180.degree. aft of the beam and others for reaching conditions of about 60.degree. to 100.degree. , an all-purpose spinnaker sail is the preferred sail of choice and is typically required to perform well in reaching and running conditions.
A well designed, all purpose spinnaker will thus be a compromise between a running spinnaker and a reaching spinnaker and eliminates the need for excessive number of spinnakers for light, medium and heavy conditions for which spinnakers have typically been designed based on nylon cloth weights of 0.5, 0.75 and 1.5 oz. per sailmaker's yard, respectively, for an average sized boat of about 37 feet in length. These weights are only nominal. Actual finished weights are always higher by the following percentage 0.5 oz.-100%; 0.75 oz.-66%; 1.5 oz.-50%. Moreover, this compromise in purpose carries over in the materials used. A running spinnaker does not have as heavy a load imposed on it as a reaching spinnaker under the same normal sailing conditions. However, a shock load imposed on a spinnaker when it collapses and refills is considerable be it experienced by a reaching or a running spinnaker.
Early spinnaker designs utilized very light woven cotton materials. However, the weight of these materials was still considerable and the spinnakers could not support broad shoulders. Other materials were tried such as silk, but until the appearance of woven nylon, the materials were not satisfactory to provide the all purpose characteristics a woven nylon fabric was able to provide.
Nylon has a combination of characteristics which renders it very suitable for making spinnakers. Nylon is a polyamide of various chemical compositions of which Nylon 6 or Nylon 66 is a typical material. Nylon has high tensile strength for a given denier and also high elasticity. Consequently, nylon has the ability to carry the high loads imposed under normal load carrying conditions and also readily meets the high shock loads which are in part absorbed by the elastic properties of nylon.
When designing a shape into this sail, an evolution of design features has also occurred. Early spinnaker designs were cross-cut such that not much of a shoulder was possible to design into the spinnaker and the smooth curves for optimum performance especially in the head of a sail could not be achieved by this design in such sail. Such sails were said to carry a "nose" in the head portion of the spinnaker. A shoulder is defined as the area of sail projecting outside of an imaginary line drawn from a midpoint of a spinnaker leach or luff to the head of the spinnaker. The leach on the shoulder forms an arc for which the imaginary line as mentioned above is a chord. A head section of the spinnaker has its bottom at the vertical point on each leach at which the leaches cease to be parallel and begin to converge area where the leach and the luff start converging.
Large shoulder, imply a large head angle. A head angle is defined as the angle formed by the leaches at the head of spinnaker sail. Early spinnaker designs based on a cross-cut design or on modified cross-cut design were not able to support a large head angle, because loads in the head section were not properly dealt with in these spinnakers. Modified cross-cut spinnakers' head panels were placed perpendicular to the leach or luff and met in a chevron fashion in a middle seam of spinnaker. Such spinnaker's were said to show an "elephant's behind". However, the modified cross-cut spinnakers were desirable because construction of these was relatively simple and the panels could be oriented perpendicular to the leaches of the sail. Such orientation was necessary to place some loads on a "thread line" and not on a bias of the woven material. For purposes of present discussion and because most spinnakers are symmetrical "leach" and "luff" is interchangeable in meaning. For asymmetrical spinnakers, leach and luff has a set meaning.
Early nylon spinnakers were made of the cross-cut or modified cross-cut type; with the understanding of load or stress maps in a spinnaker, advances were made in spinnaker designs. Typically, to accommodate the point loads in the head, tack and clew of a sail, the threads in the material should align with the lines of stress into (or out of) the point loads. Conversely, to accommodate the aerodynamic loads in the spinnaker the threads should align with the aerodynamic loads imposed on the spinnaker cloth. This understanding of stress maps first resulted in the design of radial head spinnakers which had their origin as made by Sutter Sailmakers in California and was then improved by Wally Ross of Hard Sails.
Sutter spinnakers carried the radial panel from the head all the way into the foot of the sail. Wally Ross spinnakers carried the radial gores of the material for the head into the shoulders but stopped at the center panels or center section where the aerodynamic loads were negligible and fabrication methods favored the old cross-cut panel alignment. By a cross-cut panel it is meant a panel which is carried from the luff to leach or about perpendicular from luff to leach to midline fold or center seam of a spinnaker. As mentioned above typically the panels are perpendicular to the leach and to the centerfold when the sail is folded in half vertically.
Thereafter, the radial head spinnaker evolved into a "tri-radial" spinnaker which had the radial head panels for the head, and to accommodate the point loads in the tack and clew had radiating panels from these two point load radiating to a center seam. The clew and tack radiating panels reached up to the center panels which were the old type cross-cut panels. The number of center panels could vary. "Offset tri-radial" spinnakers were developed to carry the catenaries better from the bottom tri-radial panels into the center panels by making the center panels into a number of rhomboidal shapes.
Concurrently, with the radial head spinnakers star-cut spinnakers were developed which were well suited for reaching but did not have the all purpose characteristics of the tri-radial spinnaker. Star-cut spinnakers did not have any cross-cut panels in the center section of the sail and the radiating panels from the head, tack and clew joined in the approximate center of a sail to form a "tri-star". However, star-cut spinnakers could not be made sufficiently full to be a satisfactory all purpose spinnakers.
Although many attempts have been made to supplant nylon materials by various other materials such as by very light weight polyester fabrics, plastic film, laminated plastic film and the like, the present light weight nylon materials are still preeminent.
Nevertheless, the first departure from typical fabric materials has been made by designing the material for the characteristics it has to possess in the particular locations in the spinnaker. Such development had its genesis in Conrad U.S. Pat. No. 4,593,639, and Conrad U.S. Pat. No. 4,708,080 which resulted in Conrad U.S. Pat. No. 5,038,700 and show the extensive art by the citation of the prior art found in these patents. The last patent addressed some of the prior art problems in spinnakers. The present invention is a further improvement based on my last patent and in addition incorporates novel combinations of materials and design features in the spinnaker.